Over the years, performance demands on commercial transport aircraft has increased in the area of noise, fuel economy and reduced weight. Various technological advances continue to improve aircraft performance in each of the above mentioned areas. For example, continuous improvement in engines leads to better fuel economy. Even more, shifting from propeller powered systems to turbofans has led to quieter aircraft, while using more composite material in primary structures has yielded weight savings.
Today, however, there is increasing interest in going back to the use of turboprop engines from turbofan engines, especially for commercial transport aircraft applications. While significant strides have been made over the years to improve the fuel economy of turbofan engines, such engines still are not as fuel efficient as turboprop engines. On the other hand, while turboprop engines enjoy a fuel efficiency advantage over turbofan engines, turboprop engines generate higher noise levels during operation. Present day turboprop engines will not likely be able to meet the increasingly stringent noise level regulations that are expected to be enacted at airports around the world over the next several years. Thus, a challenge exists in providing a turboprop engine that generates less noise than present day turboprop engines.
The increased noise associated with turboprop engines is generally due to the propeller blade tips and the vortices associated with the blade tips. This increased noise impacts the communities surrounding an airport, as well as the ambient environment within the cabin of a commercial passenger transport aircraft. The increased noise is most noticeable during takeoff, when power is highest and altitude is lowest. When power input to the propeller of a turboprop engine is large, the thrust produced by the propeller is large, and the noise thus generated is commensurately greater.
Another drawback with turboprop engines is the speed disadvantage the turboprop engines suffer when compared to turbofan engines. In some instances, depending on the route being flown, this may not be a concern. For example, on shorter flight routes, the desired number of missions being flown per day may still be within the performance capabilities of an aircraft employing turboprop engines. However, on longer flight routes, the increased length of time needed for a given flight would likely be viewed negatively by paying passengers, if the aircraft was a commercial passenger transport aircraft.
What would be highly desirable is a new propulsive apparatus for powering transport aircraft that combines the benefits of better fuel economy of turboprop engines, with the reduced noise of a turbofan engine.